Best bet is just go to the dealer and buy some genuine fluid, its not expensive and they carry it on the shelf.
The only other compatible fluid I know of is the Febi equivalent that GSF sell.
If you're not sure how old or what the fluid is in the system already, I recommend a full drain and flush. the wrong fluid is the most common cause of leaks and failure in the Audi hydraulic systems - you will be surprised how much ugly looking shit comes out
